1. Field of the Invention
The invention relates to a process for controlling the temperature of a fiber spinner used during the manufacture of so-called insulating glass fibers. The invention also relates to an apparatus suitable for performing this process. It is applied in particular to the automation of lines producing glass wool used principally in the composition of heat and/or sound insulating products.
2. Description of the Related Art
So-called insulating glass fibers are currently produced by internal centrifuging, i.e., by introducing a thin stream of molten glass into a centrifuger, which is also known as a fiber spinner rotating at high speed and having on its periphery a very large number of orifices. Under the effect of centrifugal force, the glass is projected through these orifices in the form of filaments. In addition to the centrifugal force there may also be a process of drawing by a high temperature and velocity gaseous current, emitted tangentially to the perforated wall of the centrifuger. In these techniques the centrifuger--of which the diameter may optionally exceed one meter--is very highly stressed by forces of a mechanical (high rotational velocity), thermal (glass at approximately 1000.degree. C.) and chemical (corrosion by the glass) origin. The quality of the fibers produced depends very strictly on the correct operation of the centrifuger, i.e., on its good general condition and adherence to the reference values with respect to velocity and temperature.
There is hardly any reason for the velocity to be modified by disturbances during manufacture and it may be mastered completely independent of all the other parameters if the shaft is driven by an asynchronous motor controlled in a suitable manner. The reference value given for the rotational velocity may thus be considered as being strictly adhered to.
In contrast, the temperature of the centrifuger is sensitive to a large number of factors including, for example, the action of internal burners heating the interior of the centrifuger and any additional heating means, a magnetic induction heater directed more particularly at the base of the centrifuger, the temperature of the glass, the flow rate of the glass, the temperature of the gaseous drawing current emitted in the immediate vicinity of the centrifuger, the relatively hot atmosphere prevailing around the centrifuger, the relatively intense cooling owing to the relatively high rotational velocity, and the centrifuger itself which may become deformed after a given amount of operating time and may consequently react differently to the effect of heating by the burners.
As indicated above, unsatisfactory temperatures cause large-scale disturbance of the fiber-drawing process. Thus if the centrifuger is too cold, devitrification may begin which renders the glass unsuitable for fiber-drawing; a centrifuger which is too hot and is at the thermal fracture limit and may moreover lead to the formation of non-fibered portions or extremely fine fibers which are undesired, owing to the glass being too highly fluid.
In order to control the temperature of the centrifuger it is proposed in U.S. Pat. No. 4,392,879 to place thermocouples on the internal wall of the centrifuger and to transform the electric signal received into an electro-magnetic wave analyzed by a fixed receiver which is not rotationally integral with the centrifuger. The electrical signal received then serves as a parameter for regulating the annular burner for the gaseous drawing process located just on the periphery of this centrifuger. In this technique, the number of points analyzed on the surface of the centrifuger is of necessity restricted relative to the height of the strip and in particular relative to the total length of the circumference of the centrifuger. In addition the internal temperature is not always highly instructive since its value may in fact be more characteristic of the temperature of the molten glass accumulated in the form of a reserve than of the actual temperature of the centrifuger. In addition, a thermocouple may become detached from the internal wall and then indicate the temperature of a point other than that which it is supposed to analyze. The temperature of a centrifuger is not constant over the entire height of the strip, if only because the glass does not always melt completely uniformly; these irregularities in temperature are thus considered to be normal provided they remain within defined limits. Delocalized measurements, i.e., incorrect measurements which are interpreted as correct, may lead to random control of the annular burner.
There is further disclosed in EP-Bl-219,433 a process for manufacturing glass fibers in which the maximum temperature of the centrifuger is determined by continuously measuring the radiation emitted by the strip during a reciprocating movement over the entire height of the strip. The maximum temperature is subsequently used as a value for regulating the burner. The equipment is less fragile than in the previous case. Furthermore, the temperature is effectively measured at the hottest point and not at the point which is assumed to be the hottest. It is thus possible to avoid overheating wherever it occurs and not only at the point where it is expected. Moreover, it is the external temperature which is measured and not the internal temperature of the centrifuger strip, the external temperature being more representative of the temperature of the fiber-drawing process by centrifuging.
It appears that the apparatus disclosed in EP-Bl-219,433 is most advantageous during the phase when the fiber-drawing process starts, i.e., the temperature of the centrifuger must be gradually increased without creating thermal shock. The apparatus enables the process to be systematically below critical temperatures and the theoretical heating profile of the part is properly respected.
During the fiber-drawing phase it has been noted that it is not always desirable to modify the properties of the drawing gas since the quality of the fibers produced is then modified, the relation between the temperature of the centrifuger and the temperature of the drawing gases not being purely straightforward but being influenced by other parameters such as the deformation of the centrifuger which heats up as it approaches the burner, for example. In addition, it has been noted above that the temperature of the centrifuger is not normally constant over the entire height of the strip. Although differences of low amplitude are considered normal, differences which are too great will damage the centrifuger and the quality of the fibers produced insofar as they result in abnormal operation of the apparatus.
In addition, the apparatus disclosed in U.S. Pat. Nos. 392,879 and EP-Bl-2l9,433 are both practically ineffective as regards the noting of defects such as the beginning of cracking of the centrifuger or even deformation owing to a lower part of the centrifuger being warped towards the exterior. These defects may cause the centrifuger to rupture in the fiber-drawing hood with a considerable risk of igniting or large scale damage, or even danger of injury for any workers in the vicinity.